Basal Body Temperature 101

In this post, I’m going to explain Basal Body Temperature and tell you why it’s important.

A little background:

When you are sleeping or very relaxed the core temperature of your body drops. This is why after getting a relaxing massage you might feel chilly. This low, resting temperature is called your basal body temperature, (BBT) and it can tell you a lot about your fertility.

BBT is measured the moment you wake up, after a good nights sleep, BEFORE you get out of bed, or do anything at all. Eating, drinking, chatting with your sweetheart, getting up to brush your teeth, etc. can all raise your temperature from its resting state and obscure your BBT reading.

Basal body temperature can be measured orally, vaginally or rectally. Measuring BBT orally is the standard and is just as reliable as vaginal or rectal measurements for the purposes of fertility charting. Why stick thermometers in those sensitive areas if you don’t have to? (unless you’re into that kind of thing)

Why it’s important:

If you are trying to get pregnant, it’s a good idea to know as much as possible about the state of your fertility. BBT doesn’t tell you when you are becoming fertile, (that’s what cervical fluid is for) what it does is confirm ovulation, which no other method easily available to you and without extensive doctor visits can. It’s easy, fast, inexpensive and accurate. Your BBT chart can confirm that you have ovulated, which urine test strips (OPKs) and ferning microscopes can not do. And since the first thing you’ll want to know if you’re trying to conceive is “Do I Ovulate?”, this is important information.

Another thing BBT can tell you is the length and health of your luteal phase. If your luteal phase is too short, or you don’t produce enough progesterone, this can cause big problems when trying to conceive. Your BBT chart will show you how long, and how healthy your luteal phase is. Again, this is very important information, not easily obtained any other way.

How it Works:

Women who have natural menstrual cycles have a bi-phasic BBT pattern. Bi = two , phasic = phase, so that means your temperatures have two phases. The first phase of your cycle has low temperatures, while the second phase of your cycle has higher temperatures.

The first phase is called the follicular phase. This phase starts on the first day of your period, each cycle, and ends at ovulation. Your BBT will show a set of low temperatures during this time, usually ranging from 97.0 to 97.6.

After ovulation is the luteal phase. Progesterone, produced by the corpus luteum during your luteal phase raises your core body temperature. During your luteal phase your BBT will rise to higher temperatures, usually ranging from 97.7 to 98.3. The range of temperatures can vary from woman to woman, as everyone is unique. But the important thing to remember is that if you are cycling naturally, (not on hormonal contraceptives) your temperature chart will show this bi-phasic pattern.

BBT charting only works in women who are cycling naturally. Birth control pills, and other hormonal contraceptives prevent pregnancy by tricking your body into thinking it has already ovulated, so that you will not release another egg. Because progesterone raises your temperature, if it’s in your body consistently, as is the case with hormonal contraceptives, you will have no natural bi-phasic pattern to chart: your temperatures will always be high.

At the end of your luteal phase your temperature will drop and you will get your period. This is the beginning of your next follicular phase. Noticing the drop in temperature at the end of your luteal phase is a great way to confirm you’ll probably get your period that day.

In summary, BBT is the temperature of your body at rest, unaltered by any other factors. It can be used to reliably confirm ovulation because progesterone, which is only produced during the luteal phase, raises your core body temperature. Learning about your own BBT pattern can help you identify fertility issues and help you get pregnant sooner.

(Want a highly accurate basal thermometer that's fast, fits comfortably in your mouth, and doesn't beep? Try Wink, a bluetooth basal thermometer that automatically syncs your temperatures and the time your temperature was taken to the Kindara app.)